Drilling deep wells, say over 9,000 ft. with water-based drilling fluids and without setting well casing to prevent drill pipe sticking, is a long-standing problem. Particularly in off-shore drilling, numerous deep wells are usually drilled from a single stationary platform with a work area generally less than 1/4 acre. Thus, the wells must be directionally drilled ("whip-stocked" or "jet deflected") at relatively high angles from vertical to reach substantial distances away from the single platform. In this way petroleum may be produced from formations covering substantial underground areas including multiple producing intervals.
In general, it is most economical to drill such wells using a water-based drilling fluid which lubricates and flushes rotary drill bit cuttings from the bore hole, but more particularly, provides hydrostatic pressure or head in the well bore to control pressures that may be encountered in a petroleum-containing formation. Such hydrostatic head prevents "blow-out" or loss of gas or oil into the well during drilling. Further, the drilling fluid contains solid materials that form a thin mud cake on the wall of the well bore to seal any permeable formation traversed by the well during deeper drilling. Such water-based drilling fluids, including sea water, are substantially cheaper than the alternative oil-based fluids from the standpoint of original cost, maintenance and protecting the ocean environment.
It has long been known that one of the primary causes of drill string "sticking" is the effect of differential pressure between the hydrostatic head in the well bore and any porous, low-pressure earth formations through which the drill string passes. Under such conditions, the pressure difference presses the drill pipe against the bore hole wall with sufficient force to prevent pipe movement. This occurs because the density or weight of the drilling fluid in the well bore creates a hydrostatic pressure against the pipe that is substantially greater than that in a porous earth formation traversed by the well bore. This is due to the filtrate (water in the drilling fluid) flowing through the well bore wall and the desirable "mud cake" into the low pressure earth formation. This condition may occur in the drill collar section of the drill string which is used to apply weight to the bit directly above the drill bit, but apparently more frequently, occurs at shallower depths where return mud flow around the smaller diameter drill string is less turbulent and hence relatively laminar. Thus, where the drill pipe lies close to one side of the well bore, as in slant holes, higher differential pressure across the drill pipe increases its adherence to the side of the well bore. In a worst case, this results in differential pressure sticking of the drill string.
Correction of drill string sticking conditions usually requires a decrease in the drilling fluid pressure in the well either by reducing the hydrostatic head of the drilling fluid or increasing solids content of the fluid to reduce filtrate loss, with subsequent building of a thicker filter cake to increase the pipe contact area. Alternatively, sticking can sometimes be avoided by using smaller diameter drill pipe or fewer drill collars in the weight assembly above the bit. The problem of differential pipe sticking is frequently severe where a well encounters over-pressured formations. In such wells, the formation pressure exceeds the pressure normally expected due to hydrostatic head alone at that depth. In such wells passing through over-pressured formations, the counterbalancing hydrostatic pressure in the well cannot be reduced safely at deeper depths. However, such increased pressures on deeper formations may substantially increase the risk of fracturing the formation, with accompanying loss of drilling fluid from the well into the fracture, and creating potential well blow-out.
It is also known that frequently a drill string may stick in a drilling well because of mechanical problems between the drill string and the well bore itself. Such a condition can sometimes occur in what is known as the "keyseat effect". That is, a keyseat is created when the drill string collar or a pipe joint erodes a circular slot the size of the drill pipe tube or tool joint outside diameter in one side of the larger circular bore hole, as originally cut by the drill bit. Such a slot can create greatly increased friction or drag between the drill string and the earth formation and result in seizure of the drill collars when an attempt is made to pull the string out of the hole and the collars become wedged in the keyseat. Such problems can also be created by excessive weight on the drill string so that the drill string buckles in the lower section and particularly where the bore hole is at a high angle, say in excess of 60.degree. from vertical, or the well bore includes more than one change of direction, such as an S-curve or forms one or more "dog-legs" between the drilling platform and the drill bit. It is also known that in mechanical sticking of a drill string, earth formations around the well may be sufficiently unstable so that the side wall collapses into the well bore and thereby sticks the pipe.
It is estimated that the cost to the petroleum industry for stuck drill pipe in drilling wells is on the order of one-hundred to five-hundred million dollars per year and the cost to rectify each occurrence can be on the order of $500,000. The extent of each pipe sticking problem generally depends upon the amount of time the operator is willing to "wash over" the stuck section of the drill pipe (after unthreading and removal of the unstuck portion), or to "fish" by otherwise manipulating the drill string. Correction may also include spotting or completely replacing the water-based drilling fluid with oil-based drilling fluid. Failure to free the drill string results either in abandoning the well bore or side tracking the bore hole above the stuck point. This may include loss of the drill bit, collars and stuck lengths of pipe in the bore hole.
The problem of sticking pipe has been described in numerous publications in the literature, particularly as it relates to differential sticking of the well bore, that is, adherence of the drill string against a porous formation so that there is no circulation of drilling fluid around one side of the drill string. As noted above, such sticking occurs generally where the drilling fluid contains too few solids or fluid loss control agents allowing increase in the thickness of the mud, or filter cake, between the drill string and the side of the well bore due to liquid loss from the drilling fluid into a porous formation. Such literature is primarily directed to methods to avoid differential sticking by assuring that the drilling fluid is tailored to match the earth formations penetrated by the well bore.
In drilling deep wells, where intimate knowledge of the formations is not available, and particularly where low pressure formations are encountered, it is difficult to predict and take corrective or preventive action prior to such drill pipe sticking. Further, while these problems can be avoided by deeper casing of the bore hole around the drill string, such casing is expensive and in general undesirable because it limits formation evaluation with conventional well logging tools. Expense is also a primary reason that oil-based drilling fluid is not desirable, unless essential to the drilling operation. Many formation evaluation or well logging tools depend upon the use of water-based drilling fluids because such fluids are electrically conductive through the earth formation, rather than insulative, as in the case of oil-based drilling fluids. Since the cost of preventive action can be exorbitant as compared to conventional drilling systems, it is highly desirable, if at all possible, to drill with conventional water-based drilling fluids while still avoiding drill pipe sticking.
Examples of patents that disclose methods and apparatus to avoid or remedy stuck pipe include the following:
U.S. Pat. No. 4,428,441--Delinger proposes the use of non-circular or square tool joints or drill collars, particularly in the drill string directly above the drill bit. Such shape assures that circulation is maintained around the drill pipe and reduces the sealing area between the pipe and the side wall where the differential pressure may act. However, such tools are expensive and not commonly available. Further, they may tend to aggravate the keyseat problem in relatively soft formations since the square edges of such collars may tend to cut the side wall in high angle holes.
U.S. Pat. No. 4,298,078--Lawrence proposes using a special drill section directly above the drill bit to permit jarring the drill bit if the pipe tends to stick. Additionally, valves in the tool may be actuated to release drilling fluid around the drill string to assist in preventing or relieving stuck drill string condition.
U.S. Pat. No. 4,427,080--Steiger is directed to binding a porous layer on the outside of the drill string. Such a coating is stated to prevent differential pressure sticking of the pipe by increasing liquid flow around the drill string.
U.S. Pat. No. 4,423,791--Moses discloses avoiding differential sticking by use of glass beads in the drilling fluid to inhibit formation of a seal by the filter cake between the drill string and the well bore adjacent a low pressure zone.
While it has been proposed heretofore to statistically study the probability of relieving differential sticking of a drill pipe, such statistical analysis has been directed to the problem of estimating minimum soaking time and maximum fishing time that may be economically devoted to unsticking the stuck drill pipe. Such a procedure is disclosed in an article published at the Offshore Technology Conference of 1984 entitled "Economic and Statistical Analysis of Time Limitation for Spotting Fluid in Fishing Operations" by P. S. Keller et al. "Stickiness Factor--A New Way of Looking at Stuck Pipe", IADC/SPE paper 11383, 1983 Drilling Conference, pages 225-231 by T. E. Love is directed to a statistical study of "stickiness factor" for evaluating the probability of freeing stuck pipe by use of an empirical formula that evaluates several significant variables in drilling a well, namely, the length of open hole, mud weight, drilling fluid loss, and length of the bottom hole assembly. The formula was developed from wells in which drill pipe had become stuck and those in which drill pipe had not stuck by cross-correlation of 14 primary parameters measured in connection with drilling wells in a given area of the Gulf of Mexico. The primary purpose of the formula is to determine the chance of freeing stuck pipe and in guiding the well by controlling only the chosen variables used in the empirical formula. No suggestion is made to use statistical analysis of such differentially stuck walls along with mechanically stuck wells or to determine the probabilities of modifying only certain measured well variables to divert well drilling conditions from either of such stuck well conditions to a non-stuck condition.
Studies have also been reported by M. Stewart (Speech to Society of Petroleum Engineers, New Orleans Chapter, New Orleans, La., 1984) on the problem of setting casing at particular depths with statistical studies of differentially stuck pipe, particularly in the Gulf Coast, in wells that encounter over-pressure formations to avoid inadequate bore hole hydrostatic head on such formations or fracturing of lower pressure formations, as discussed above.